Solid combustible wastes have always been generated by industry. Many of such wastes, because of their flammable or toxic character, are categorized by applicable environmental regulations as "hazardous wastes". Prior to governmental regulation of the disposal of such materials, they were disposed of by dumping in landfill operations. Significant environmental damage has been caused by those practices. With recently enacted environmental regulations imposing severe restrictions on landfill-type disposal of hazardous wastes, the only viable means for their safe disposal has been by thermal treatment, typically at high cost in specialized hazardous waste incinerators equipped with extensive emission control devices.
Cement kilns have received favorable review from both federal and state environmental regulatory agencies as providing ideal conditions for disposal of combustible waste materials. Not only does the burning of hazardous waste in operating kilns allow recovery of energy values from hazardous wastes, but also, because of their high operating temperatures, long residence times and their ability to provide favorable conditions for the chemical combination of inorganic residues into the active compounds of portland cement, such operations provide ideal conditions for environmentally sound disposal of combustible hazardous waste materials.
However, because of problems unique to the handling and burning of solid hazardous waste in operating kilns, regulation compliant disposal of hazardous wastes in operating kilns has been limited to combustible liquid or "pumpable" hazardous waste. Liquid waste materials are easily blended with each other and with conventional fuels to provide homogeneous liquids that can be burned in the gaseous phase at the firing end of the kiln with little or no modification of kiln burner configuration. Solid hazardous wastes, however, can occur in a multiplicity of forms, from hard crystalline solids to viscous, sticky sludges. They are not easily blended and they present significant engineering challenges for their safe handling and delivery into rotary kilns. Further, the burning of combustible solids in the firing chamber of a kiln faces other practical problems. Hazardous waste solids are not easily dispersed into the flame of the burning primary fuel. If waste solids are charged into the primary combustion zone, they will necessarily come into contact with the mineral bed at a very critical time in the clinker-forming process. It is important for the formation of quality clinker, both in terms of color and performance, that oxidizing conditions be maintained in the clinker-forming zone of the kiln. Charging combustible solids onto the forming clinker at temperatures in excess of 1300.degree. C. can create reducing conditions in the forming clinker and adversely affect cement quality.
Before promulgation of existing EPA air quality regulations, it was a practice to charge combustible solid waste into the cold end of the kiln with the to-be-processed mineral materials. Such practices continue in countries where emission standards are not in effect or not enforced. Today, however, without added expensive emission control equipment, combustible solid hazardous waste cannot be charged to the cold end of the kiln and still allow the kiln to operate in compliance with environmental regulations controlling kiln emissions. Combustible solid hazardous waste by definition can contain a wide range of volatile organic substances. Combustible hazardous waste solids, added with the raw material to the "cold" or upper end of a conventional kiln, without combustion gas afterburners or other emission control equipment, results in unacceptable levels of hydrocarbon emissions. As the raw material and waste solids move down the kiln cylinder to higher and higher temperature zones, volatile components are driven off into the effluent gases at temperatures below those required for thermal degradation of the volatilized components. The result is discharge of a significant amount of the volatile compounds into the atmosphere. Particulate residue fuels with low volatiles content can be added either to the mineral material introduced at the cold end of the kiln or to the calcining zone with efficient energy value usage and apparently without notable problems with hydrocarbon emission levels. See, for example, U.S. Pat. No. 4,022,629 and references cited therein.
Combustible hazardous waste solids represent a significant potential source of inexpensive energy to the energy intensive mineral processing industry. However, concerns about waste handling, plant engineering, end-product quality and emission control has deterred kiln operators from taking advantage of the inexpensive energy values available through burning of hazardous waste solids. That has been true particularly in view of the availability of, and the environmentally sanctioned use of, hazardous waste liquids as inexpensive alternate fuels for kiln operations. However, with the promulgation of environmental regulations imposing severe restrictions on land disposal of solid hazardous wastes, and the limited availability (and high costs) of EPA-approved complete combustion facilities, there has been a significant effort directed toward the development of alternate means for safe disposal of solid hazardous wastes. The present invention evolved from that effort.
One object of this invention is to provide a safe, environmentally acceptable method for disposal of hazardous waste materials, including particularly hazardous waste solids, which contain significant levels of combustible and/or toxic organic compounds and toxic inorganic substances. It is another object of this invention to provide a method of recovering energy values of solid combustible hazardous waste materials and of using such materials to provide up to 40% or more of the energy requirements of kiln operation. The method is fully compliant with applicable environmental emission regulations, and it also allows for the most efficient use of the waste material as fuel in the process without compromising quality of the processed mineral product.
One aspect of the invention comprises a unique configuration of hazardous waste processing, packaging (containerization), and kiln charging techniques which enables kilns to use controlled quantities of hazardous wastes as supplemental fuel while maintaining compliance with environmental emission standards and minimizing the risk of personal injury to individuals handling of the waste products.
This has been practiced by preparing fuel modules of containerized hazardous wastes. Most preferably the hazardous waste is packaged in portions having energy values within a predetermined energy value range. The fuel modules are charged into an operating kiln at a point where the temperature and process conditions assure environmentally sound disposal of both volatile and non-volatile waste components and where energy and material content of the waste material most efficiently contribute to the mineral processing operation. Containers of the combustible hazardous waste are charged to the kiln at regular intervals at a point in the kiln where kiln gas temperatures range from about 950.degree. to about 1200.degree. C., more preferably from about 950.degree. C. to about 1100.degree. C. These temperatures are high enough to assure complete combustion of volatilized components but not so high that where the process is conducted in a mineral processing kiln, the presence of the combustible non-volatile portion of the charged waste in the mineral bed creates conditions detrimental to product quality.
One embodiment of the invention is an apparatus that enables the charging of solid fuel or containerized fuel through the wall of a rotating kiln cylinder. A port, preferably with a mechanical closure in the kiln cylinder wall is aligned with a drop tube inside the kiln cylinder. The drop tube prevents hot mineral material in the kiln from escaping through the port or contacting the closure. Fuel is delivered to the kiln through the port and the drop tube at predetermined times during kiln cylinder rotation.
Blending hazardous waste material to form hazardous waste homogenates prior to containerization facilitates process control and minimizes perturbation of kiln operation conditions by assuring some uniformity (from container to container) in terms of waste composition, form, energy value and combustion characteristics. Containerization of the hazardous waste, preferably as a hazardous waste homogenate, not only provides a safe and convenient means for handling and shipping of hazardous wastes, but it also seems to play an important role in the satisfactory degradation of the contained waste, particularly the volatile portion, in the kiln. Sealed containers of hazardous wastes are charged into the kiln at a point where kiln gas temperatures are high enough to decompose or complete combust volatile components driven into the gas stream. It is important that the volume of volatile components not exceed the capacity for their complete combustion in the gas stream. With the hazardous waste in sealed containers or modules, release of the volatile components of the contained hazardous waste into the kiln gas stream occurs over a period of time commensurate with the time for melting or disintegration of the container itself. Containerization of the waste therefore minimizes the potential for overloading the complete combustion capacity of the kiln gas stream with volatile organics. The result is a destruction and removal efficiency (DRE) of principal organic hazardous constituents (POHC's) of 99.99% and higher for the process.
Introduction of the containerized waste in the 950.degree.-1200.degree. zone of the kiln not only assures acceptable complete combustion of volatile components but also enables efficient use and disposal of the non-volatile components. Combustible non-volatile components are burned while in contact with the calcining mineral material allowing for high heat transfer efficiency. The inorganic components of the non-volatile residue come into immediate contact with and react chemically with the free calcium oxide forming in the mineral bed to become incorporated into the cement materials and rendered non-hazardous.